The conversion of oxygenates to olefins (OTO) is currently the subject of intense research, because it has the potential for replacing the long-standing steam cracking technology that is today the industry-standard for producing world scale quantities of ethylene and propylene. The very large volumes involved suggest that substantial economic incentives exist for alternate technologies that can deliver high throughputs of light olefins in a cost efficient manner. Whereas steam cracking relies on non-selective thermal reactions of naphtha range hydrocarbons at very high temperatures, OTO exploits catalytic and micro-architectural properties of acidic molecular sieves under milder temperature conditions to produce high yields of ethylene and propylene from methanol.
Current understanding of the OTO reactions suggests a complex sequence in which three major steps can be identified: (1) an induction period leading to the formation of an active carbon pool (alkyl-aromatics), (2) alkylation-dealkylation reactions of these active intermediates leading to products, and (3) a gradual build-up of condensed ring aromatics. OTO is therefore an inherently transient chemical transformation in which the catalyst is in a continuous state of change. The ability of the catalyst to maintain high olefin yields for prolonged periods of time relies on a delicate balance between the relative rates at which the above processes take place. The formation of coke-like molecules is of singular importance because their accumulation interferes with the desired reaction sequence in a number of ways. In particular, coke renders the carbon pool inactive, lowers the rates of diffusion of reactants and products, increases the potential for undesired secondary reactions and limits catalyst life.
Over the last two decades, many catalytic materials have been identified as being useful for carrying out the OTO reactions. Crystalline molecular sieves are the preferred catalysts today because they simultaneously address the acidity and morphological requirements for the reactions. Particularly preferred materials are eight-membered ring aluminosilicates, such as those having the chabazite (CHA) framework type, as well as silicoaluminophosphates of the CHA structure, such as SAPO-34. These molecular sieves have cages that are sufficiently large to accommodate aromatic intermediates, while still allowing the diffusional transport of reactants and products into and out of the crystals through regularly interconnected window apertures. By complementing such morphological characteristics with appropriate levels of acid strength and acid density, working catalysts are produced. Extensive research in this area indicates that silicoaluminophosphates are currently more effective OTO catalysts than aluminosilicates. In particular, the control of the silica to alumina molar ratio is a key requirement for the use of aluminosilicates in OTO reactions.
Molecular sieves are classified by the Structure Commission of the International Zeolite Association according to the rules of the IUPAC Commission on Zeolite Nomenclature. According to this classification, framework-type zeolites and other crystalline microporous molecular sieves, for which a structure has been established, are assigned a three letter code and are described in the Atlas of Zeolite Framework Types, 5th edition, Elsevier, London, England (2001).
One known molecular sieve for which a structure has been established is the material designated as AEI, which is a molecular sieve having pores defined by two sets of generally perpendicular channels each having a cross-sectional dimension about 3.8 Å. Molecular sieves of the AEI framework type do not exist in nature, but a number of aluminophosphates and metalloaluminophosphates having the AEI framework type have been synthesized, including SAPO-18, ALPO-18, and RUW-18. Moreover, because of their small pore size, AEI-type molecular sieves have been reported as suitable catalysts for a variety of important chemical processes, including the conversion of oxygenates to olefins. See, for example, U.S. Pat. No. 5,095,163, incorporated herein by reference.
Regular crystalline molecular sieves, such as the AEI and CHA structure type materials discussed above, are built from structurally invariant building units, called Periodic Building Units, and are periodically ordered in three dimensions. However, in addition to pure phase molecular sieves, disordered structures showing periodic ordering in less than three dimensions are also known. One such disordered structure is a disordered planar intergrowth in which the building units from more than one structure type are present. Of particular interest are the intergrowths of containing both AEI and CHA structure type materials since these molecular sieves promise to be particularly attractive catalysts for OTO reactions.
International Patent Publication No. WO 02/70407, published Sep. 12, 2002, and incorporated herein by reference, discloses a silicoaluminophosphate molecular sieve, now designated EMM-2, comprising at least one intergrown phase of molecular sieves having AEI and CHA framework types, wherein said intergrown phase has an AEI/CHA ratio of from about 5/95 to 40/60 as determined by DIFFaX analysis, using the powder X-ray diffraction pattern of a calcined sample of said silicoaluminophosphate molecular sieve. Synthesis of the intergrown material is achieved by mixing reactive sources of silicon, phosphorus, and a hydrated aluminum oxide in the presence of an organic directing agent, particularly a tetraethylammonium compound. The resultant mixture is stirred and heated at a rate of 22-35° C./hour to a crystallization temperature, preferably from 150° C. to 185° C., and then maintained at this temperature under stirring for between 2 and 150 hours. The resultant EMM-2 material is shown to be an active and selective catalyst for converting methanol to light olefins.
U.S. Pat. No. 6,334,994, also incorporated herein by reference, discloses a silicoaluminophosphate molecular sieve, referred to as RUW-19, which is also said to be an AEI/CHA mixed phase composition. In particular, RUW-19 is reported as having peaks characteristic of both AEI and CHA structure type molecular sieves, except that the broad feature centered at about 16.9 (2θ) in RUW-19 replaces the pair of reflections centered at about 17.0 (2θ) in AEI materials and RUW-19 does not have the reflections associated with CHA materials centered at 2θ values of 17.8 and 24.8. DIFFaX analysis of the X-ray diffraction pattern of RUW-19, as produced in Examples 1, 2, and 3 of U.S. Pat. No. 6,334,994, indicates that these materials are characterized by single intergrown phases of AEI and CHA structure type molecular sieves with AEI/CHA ratios of about 60/40, 65/35 and 70/30. Again RUW-19 is reported to be active as a catalyst in the production of light olefins from methanol.
U.S. Patent Application Publication No. 2005/0233895, published Oct. 20, 2005, discloses a silicoaluminophosphate molecular sieve that comprises first and second intergrown phases of a CHA framework type and an AEI framework type, wherein said first intergrown phase has an AEI/CHA ratio of from about 5/95 to about 40/60 as determined by DIFFaX analysis, the second intergrown phase has an AEI/CHA ratio of about 30/70 to about 55/45 as determined by DIFFaX analysis and the molecular sieve has a silica to alumina molar ratio (Si/Al2) from about 0.13 to about 0.24. In Example 1, the molecular sieve was produced by heating a mixture of phosphoric acid, demineralized water, tetraethylammonium hydroxide solution, Ludox AS 40 (40% silica), and alumina having the following composition:0.12SiO2/Al2O3/P2O5/TEAOH/35H2Oin a stainless steel autoclave at a rate of 20° C./hour to 165° C. and then holding at this temperature for 60 hours.
One of problems involved in the synthesis of aluminophosphate and metalloaluminophosphate molecular sieves, such as those comprising AEI structure type materials and, in particular, those comprising intergrowths of both AEI and CHA structure type materials, is the low product yield typically obtained in the synthesis process. For example, existing synthesis routes often achieve yields as low as 15 wt %, based on the solids added to the reaction mixture. Moreover, although a variety of methods have been proposed to increase yield, such as increasing crystallization temperature, as well as prolonging time at the crystallization temperature, these methods are often accompanied by an undesirable increase in the production of impurity phases. There is, therefore, significant interest in finding alternative methods of increasing product yield. According to the present invention, it has now been found that product yield in the synthesis of aluminophosphate and metalloaluminophosphate molecular sieves comprising AEI structure type materials can be increased by decreasing the rate of heating to the desired crystallization temperature, especially when the water content of the reaction mixture is also reduced. 
U.S. Patent Application Publication No. 2007/0203385, published Aug. 30, 2007, discloses that the attrition resistance index of a metalloaluminophosphate molecular sieve can be adjusted by control of its morphology and size index (MSI). In particular, the Examples show that, when crystallizing an AEI/CHI intergrown silicoaluminophosphate molecular sieve from a mixture of water, 85% phosphoric acid, colloidal silica, pseudoboehmite, and tetraethylammonium hydroxide (TEAOH) having the composition:0:10SiO2:Al2O3:P2O5:TEAOH: 34H2O,reducing the heating rate to the crystallization temperature from 13° C./hour to 6.4° C./hour can reduce the MSI of the product. No information is provided as to the effect of the heating rate on the yield of the molecular sieve.
U.S. Pat. No. 7,008,610 discloses synthesis of a crystalline aluminosilicate material having an AEI framework type in the presence of a halogen compound, such as a fluoride, and N,N-diethyl-2,6-dimethylpiperidinium cations as the structure directing agent. In Example 1, a yield of AEI framework type molecular sieve of 28.3 wt %, based on the weight of the dry gel was obtained, although it will be seen that the molecular sieve produced by this patent is substantially free of framework phosphorus.